1. Field of the Invention
This invention relates to a liquid crystal display device and its driving method, more specifically, to a liquid crystal display device and its driving method employing the horizontal line inverting method.
2. Description of the Related Art
In general, the AC driving method is employed in liquid crystal display devices. This is because the useful life becomes short if the liquid crystal layer is driven with DC voltage. Also well known as a driving method for reducing flicker during AC driving is the horizontal line inverting method that inverts polarity at every unit horizontal synchronization cycle (1H cycle).
For example, as shown in FIG. 1, a liquid crystal display device using this prior art driving method has a switching circuit 107 that switches the outputs from a first standard voltage generating circuit 106a that generates positive-polarity standard voltage and a second standard voltage generating circuit 106b that generates negative-polarity standard voltage, in synchronization with the synchronization signal provided by a control circuit 101. The output of the switching circuit 107 is connected in common to a plurality of horizontal drivers 103 connected to the signal lines of a liquid crystal panel 105.
The control circuit 101, responding to the input data for the image displayed on the liquid crystal panel 105, makes the horizontal drivers 103 apply the voltage provided from the first standard voltage generating circuit 106a to the liquid crystal panel 105, corresponding to the input gradation data for a unit 1H cycle. During the subsequent 1H cycle, it makes the horizontal drivers 103 apply the voltage provided from the second standard voltage generating circuit 106b to the liquid crystal panel 105.
Further, the control circuit 101 makes a common voltage generating circuit 104 apply a common voltage to the liquid crystal panel 105. To the electrode of each pixel in the liquid crystal panel 105, the horizontal driver 103 supplies a signal voltage corresponding to the gradation data when a vertical driver 102 has chosen a scanning line. Meanwhile, the common voltage generating circuit 104 provides the common voltage for the common electrode opposing this pixel electrode. Then an image of gradation corresponding to the voltage gap between the pixel electrode and the common electrode is displayed on the liquid crystal panel 105. This common voltage is inverted at every 1H cycle and supplied to the liquid crystal panel 105 in order to enlarge the effective voltage applied to each pixel of the liquid crystal panel 105. The AC driving of the liquid crystal panel is performed by this line inversion at every 1H cycle.
The gradation-γ correction voltage relation of a liquid crystal display device is shown in FIG. 2A. The dotted line represents the gradation-γ correction voltage relation that does not take into account the applied voltage-transmittance property of the liquid crystal layer, while the solid line represents the gradation-γ correction voltage relation incorporating correction that has taken into account the applied voltage-transmittance property of the liquid crystal layer. Since the applied voltage-transmittance property of the liquid crystal layer is not represented with a straight line or is not linear, driving voltage is applied to the liquid crystal panel based on the gradation-γ correction voltage relation denoted with the solid line in the diagram in order to realize gradation display corresponding to the input data in actual liquid crystal display devices.
If γ-correction voltage is applied to the liquid crystal panel 105 of the prior art liquid crystal display device shown in FIG. 1 based on the gradation-γ correction voltage relation represented by the solid line, the applied voltage will be VF for gradation X1, while VG for gradation X2 during the following 1H cycle. Then the effective voltage applied to the liquid crystal layer of the liquid crystal panel 105 will be |VF−VC| and |VG−VC|, respectively. Note that VC represents the common potential supplied to the common electrode opposing the pixel electrode. As a result, the effective voltage levels(F, G) differ from each other between a 1H cycle and the subsequent 1H cycle, as shown in FIG. 2B. This is the cause of flicker.
Besides, the circuit structure becomes complex in the prior art liquid crystal display device shown in FIG. 1 because the switching circuit 107 selects either standard voltage generating circuit 106a or 106b each generating positive- or negative-polarity standard voltage so as to supply standard voltage to the horizontal drivers 103. Also because the power source voltage Vcc for the standard voltage generating circuits 106a, 106b is very high, the switching circuit 107 must withstand high voltage. Then the device cost will be high.